Gas sensor and method of manufacturing the same

Abstract

To provide a gas sensor capable of stably detecting a gas component in a gas phase.SOLUTION: A gas sensor is provided, comprising a transducer and a sensitive membrane provided on a surface of the transducer, the sensitive membrane being composed of a zeolite-imidazole structure and also containing an imidazole-based compound in an amount greater than 0.3 mass%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a gas sensor capable of detecting gas components in a gas phase and a method for manufacturing the same. [Background technology]

[0002] Gas sensors using a sensitive material that adsorbs volatile organic compounds (VOCs) such as acetone have been known for some time. For example, International Publication No. 2016 / 121155 (Patent Document 1) discloses a sensor in which a porous or granular material is coated on a sensor body that detects a physical parameter, and the sensor detects analyte molecules based on a change in the physical parameter caused by the analyte molecules being adsorbed by the porous or granular material. The sensor also describes a membrane-type surface stress sensor using a metal-organic framework (MOF), in which MOF nanoparticles such as ZIF-7 or ZIF-8 are coated on the sensor. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2016 / 121155 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in conventional sensors using zeolite-imidazole structures (ZIF), ZIF nanoparticles are coated on the sensor, so the film formed on the sensor is a particle film, and the contact area between the sensor surface and the ZIF nanoparticles is small. The bonding strength between the sensor surface and the ZIF nanoparticles and between the ZIF nanoparticles is weak, so the signal is easily affected by minute changes in atmospheric conditions (gas flow, temperature, pressure, etc.), resulting in unstable signals from the sensor and unstable detection of gas components in the gas phase.

[0005] The present invention has been made in view of the problems associated with the prior art, and has as its object to provide a gas sensor capable of stably detecting gas components in a gas phase, and a method for manufacturing the same. [Means for solving the problem]

[0006] As a result of intensive research to achieve the above object, the present inventors have found that by forming a film using a ZIF precursor solution containing acetic acid hydrate of metal atoms as a metal source of zeolite-imidazole structure (ZIF), a sensitive film formed by ZIF and containing an imidazole-based compound can be obtained, and a gas sensor equipped with this sensitive film can stably detect gas components in the gas phase, thereby completing the present invention.

[0007] That is, the present invention provides the following aspects. [1] A gas sensor comprising a transducer and a sensitive film disposed on a surface of the transducer, The gas sensor, wherein the sensitive film is formed of a zeolite-imidazole structure and further contains an imidazole-based compound in an amount greater than 0.3 mass %. [2] The gas sensor according to [1], wherein at least a part of the zeolite-imidazole structure has an amorphous structure. [3] The gas sensor according to [1] or [2], wherein the crystallinity of the zeolite-imidazole structure determined based on the XRD diffraction pattern of the sensitive film is 0 to 29%. [4] A step of preparing a precursor solution of a zeolite-imidazole structure by mixing an imidazole compound and an acetic acid hydrate containing a metal ion in an organic solvent so that the molar ratio (imidazole compound / acetic acid hydrate containing a metal ion) is 1 / 1 or more; a step of applying a precursor solution of the zeolite-imidazole structure to a surface of a transducer to form a coating film; a step of converting the precursor of the zeolite-imidazole structure into a zeolite-imidazole structure while removing the organic solvent in the coating film, thereby forming a sensitive film formed of the zeolite-imidazole structure; A method for manufacturing a gas sensor, comprising: [5] The method for manufacturing a gas sensor according to [4], wherein in the step of forming the zeolite-imidazole structure, the organic solvent in the coating film is removed by heating at a temperature equal to or higher than the boiling point of the organic solvent and lower than the boiling point of the imidazole-based compound. [Effects of the Invention]

[0008] According to the present invention, it is possible to obtain a gas sensor capable of stably detecting gas components in a gas phase. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a scanning electron microscope photograph showing a cross section of a dried coating film of the gas sensor produced in Example 1. [Figure 2] 1 is a scanning electron microscope photograph showing a cross section of a dried coating film of the gas sensor produced in Example 2. [Figure 3] 1A is a scanning electron microscope photograph showing the surface and cross section of the dried coating film of the gas sensor produced in Comparative Example 1; [Figure 4] 1 is a gas chromatogram showing the analysis results of components extracted from the dried coating films of the gas sensors prepared in Example 2 and Comparative Example 1. [Figure 5] 1 is a graph showing an X-ray diffraction pattern of a dried coating film of the gas sensor produced in Example 2. [Figure 6] 10 is a graph showing the acetone gas response characteristics of the gas sensor fabricated in Example 2. [Figure 7] 1 is a graph showing the acetone gas response characteristics of the gas sensor fabricated in Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0010] The present invention will be described in detail below based on preferred embodiments thereof.

[0011] [Gas sensor] The gas sensor of the present invention is a gas sensor comprising a transducer and a sensitive film disposed on the surface of the transducer, wherein the sensitive film is formed of a zeolite-imidazole structure and further contains more than 0.3 mass % of an imidazole-based compound.

[0012] (transducer) The transducer converts changes in physical quantities (weight, stress, electric field) that occur when gas components are adsorbed and desorbed onto a sensor film on the surface of a substrate (metal, inorganic semiconductor) into an electrical signal, and examples of such transducers include well-known gas sensor transducers such as a quartz crystal microbalance (QCM), a membrane-type surface stress sensor (MSS), and a field-effect transistor (FET).

[0013] (sensitive membrane) The sensitive membrane is disposed on the surface of the transducer (particularly, the electrode surface) and is formed from a zeolite-imidazole structure (ZIF). The ZIF is a coordination polymer having a structure in which an imidazole compound is coordinated as an organic ligand to a metal ion, and examples thereof include ZIF-(7, 8, 67, 68, 69, 70, 78, 79, 81, 82). Such a sensitive membrane formed from ZIF can adsorb and desorb target gas components in the gas phase. For example, when a QCM sensor is used, which utilizes the principle that the mass of the sensitive membrane changes depending on the amount of adsorption, the target gas component in the gas phase can be detected by detecting the change in the vibration frequency of the transducer due to the change in mass of the sensitive membrane.

[0014] In the sensitive film, it is preferable that at least a part of the ZIF is amorphous structure.The sensitive film in which at least a part of ZIF is amorphous structure has a dense structure, so that the adhesion with the substrate surface formed on the transducer and the adhesion between ZIF amorphous particles are high, so by using the gas sensor with such a sensitive film, it is possible to obtain a stable signal from the sensor, and to stably detect the gas components in the gas phase.It should be noted that at least a part of ZIF is amorphous structure can be confirmed by the existence of a halo pattern in the XRD pattern of the sensitive film.

[0015] In addition, in the sensitive film, the crystallinity of the ZIF is preferably 0 to 29%. A sensitive film with a crystallinity of ZIF below the upper limit has a dense structure, so that the adhesion to the transducer surface and the adhesion between ZIF amorphous particles are high. Therefore, by using a gas sensor equipped with such a sensitive film, a stable signal can be obtained from the sensor, and it becomes possible to stably detect gas components in the gas phase. The crystallinity of ZIF can be calculated by calculating the crystalline peak area and the amorphous peak area based on the XRD pattern of the sensitive film, and then using the following formula: Crystallinity (%) = crystalline peak area / (crystalline peak area + amorphous peak area) × 100 It can be calculated as follows.

[0016] In addition, the sensitive film further contains an imidazole-based compound. Examples of the imidazole-based compound include 2-methylimidazole, imidazole, benzimidazole, 2-nitroimidazole, carbonyldiimidazole, 2-ethyl-4-methylimidazole, etc., and it is preferable that the imidazole-based compound is the same as that of the organic ligand of the ZIF used.

[0017] The content of such imidazole-based compounds must be greater than 0.3% by mass relative to the entire sensitive film. If the content of the imidazole-based compounds is below the lower limit, the adhesion between the sensitive film and the surface of the transducer decreases, the signal from the sensor becomes unstable, and it becomes difficult to stably detect gas components in the gas phase. Furthermore, from the viewpoint of improving the adhesion between the sensitive film and the surface of the transducer, further stabilizing the signal from the sensor, and enabling more stable detection of gas components in the gas phase, the content of the imidazole-based compounds is preferably 9.5% by mass or more.

[0018] [Method for manufacturing gas sensor] The method for manufacturing a gas sensor of the present invention includes the steps of: A step of preparing a precursor solution of a zeolite-imidazole structure by mixing an imidazole compound and an acetic acid hydrate containing a metal ion in an organic solvent at a molar ratio (imidazole compound / acetic acid hydrate containing a metal ion) of 1 / 1 or more; a step of applying a precursor solution of the zeolite-imidazole structure to a surface of a transducer to form a coating film; a step of converting the precursor of the zeolite-imidazole structure into a zeolite-imidazole structure while removing the organic solvent in the coating film, thereby forming a sensitive film formed of the zeolite-imidazole structure; The method includes:

[0019] (Precursor solution preparation step) The organic solvent used in the precursor solution preparation step is not particularly limited as long as it can dissolve the imidazole compound and the acetic acid hydrate containing metal ions and can be removed after film formation, and examples thereof include alcohols such as ethanol, methanol, and N,N-dimethylformamide.

[0020] The imidazole-based compound used in the precursor solution preparation step coordinates with metal ions to form a zeolite-imidazole structure (ZIF), and remains in the sensitive film to improve the adhesion between the sensitive film and the surface of the transducer. Examples of such imidazole-based compounds include those described in the section on gas sensors above.

[0021] The acetic acid hydrate containing metal ions used in the precursor solution preparation step is a metal source for ZIF. Examples of the metal ions include cobalt ions and zinc ions.

[0022] In the precursor solution preparation process, the imidazole compound and the acetic acid hydrate containing the metal ions are mixed and dissolved in the organic solvent to obtain a ZIF precursor solution. Mixing the imidazole compound and the acetic acid hydrate containing the metal ions in the organic solvent produces fine amorphous ZIF particles. This is thought to be due to steric hindrance caused by copolymerization of the metal ions and the imidazole compound, which inhibits ZIF crystal growth. Using such fine amorphous ZIF particles, a sensitive membrane formed of ZIF, at least a portion of which is amorphous, can be formed with a dense structure. This improves the adhesion between the sensitive membrane and the transducer surface and between the ZIF amorphous particles, further stabilizing the signal from the sensor and enabling more stable detection of gas components in the gas phase. Furthermore, the fine amorphous ZIF particles are highly dispersible in organic solvents and have excellent dispersion stability.

[0023] The mixing ratio of the imidazole compound to the acetic acid hydrate containing metal ions must be 1 / 1 or more in terms of molar ratio (imidazole compound / acetic acid hydrate containing metal ions). If the molar ratio is less than 1 / 1, the amount of the imidazole compound contained in the resulting sensitive film will be small, making it difficult to synthesize amorphous particles. Furthermore, from the viewpoints of improving the adhesion between the sensitive film and the surface of the transducer, further stabilizing the signal from the sensor, and enabling more stable detection of gas components in the gas phase, the molar ratio (imidazole compound / acetic acid hydrate containing metal ions) is preferably 1 / 1 or more.

[0024] (Coating film formation process) In the coating film forming process, the ZIF precursor solution is applied to the surface of the transducer to form a coating film. The method for applying the precursor solution is not particularly limited, and examples thereof include spin coating, dip coating, spraying, printing, and stamping. The precursor solution is preferably diluted or concentrated to a viscosity suitable for the coating method used.

[0025] (Sensor film formation process) In the sensitive film formation process, the ZIF precursor is converted to the ZIF while removing the organic solvent from the coating film, thereby obtaining a sensitive film formed from the ZIF. Since the ZIF precursor is fine amorphous ZIF particles, a sensitive film with a dense structure is formed. This sensitive film with a dense structure has excellent adhesion to the surface of the transducer and between the ZIF amorphous particles, which stabilizes the signal from the sensor and enables stable detection of gas components in the gas phase. In addition, since the obtained sensitive film contains a predetermined amount of imidazoline compounds, adhesion to the surface of the transducer is improved, the signal from the sensor is further stabilized, and gas components in the gas phase can be more stably detected.

[0026] There are no particular limitations on the method for removing the organic solvent from the coating film, but it is preferable to heat the coating film under reduced pressure. By heating under reduced pressure, the organic solvent can be efficiently removed, and the growth of the ZIF amorphous particles promotes sintering between the ZIF particles, improving the bonding strength between the ZIF particles. This results in the formation of a sensitive film with a dense structure. This sensitive film with a dense structure has excellent adhesion to the surface of the transducer and between the ZIF amorphous particles, so the signal from the sensor is stable and it is possible to stably detect gas components in the gas phase.

[0027] The heating temperature is preferably a temperature equal to or higher than the boiling point of the organic solvent and lower than the boiling point of the imidazole compound. If the heating temperature is lower than the boiling point of the organic solvent, the organic solvent may not be sufficiently removed, whereas if the heating temperature is higher than the boiling point of the imidazole compound, the amount of the imidazole compound remaining on the sensitive film will be too small, reducing the adhesion between the sensitive film and the surface of the transducer, making the signal from the sensor unstable and making it difficult to stably detect gas components in the gas phase. [Example]

[0028] The present invention will be described in more detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0029] Example 1 Cobalt acetate tetrahydrate (Co(COOH)2·4H2O, 622.7 mg, 2.5 mmol) was added to ethanol (25 ml) and stirred to dissolve. 2-Methylimidazole (821 mg, 10 mmol) was then added and stirred at room temperature for 12 hours to prepare a precursor solution of cobalt-organic framework (ZIF-67) in which amorphous particles were highly dispersed in ethanol. This ZIF-67 precursor solution maintained the high dispersion state of the amorphous ZIF-67 particles even after standing at room temperature for 3 months, demonstrating excellent particle dispersion stability.

[0030] Next, the obtained ZIF-67 precursor solution was concentrated by partially evaporating the ethanol at room temperature until the viscosity reached 1.2 mPa·s. After that, it was sprayed onto the Au electrode surface of the QCM element 50 times using an electrostatic spray method at an applied voltage of 12 kV. The obtained coating was then heated in a vacuum sample dryer at 150°C for 1.5 hours and vacuum dried to prepare a QCM element (gas sensor) with a dried coating on the Au electrode surface.

[0031] Example 2 A QCM element (gas sensor) with a dried coating film on the surface of an Au electrode was prepared in the same manner as in Example 1, except that the ZIF-67 precursor solution was concentrated until the viscosity reached 9.0 mPa·s.

[0032] (Comparative Example 1) Cobalt nitrate hexahydrate (Co(NO3)2·6H2O, 727.6 mg, 2.5 mmol) was dissolved in methanol (25 ml) and stirred. 2-Methylimidazole (821 mg, 10 mmol) was then added and stirred at room temperature for 12 hours, resulting in the precipitation of ZIF-67 particles. The ZIF-67 particles were collected by centrifugation, washed several times with methanol, and then vacuum-dried in a vacuum sample dryer at room temperature for approximately 1 hour and then at 200 °C for approximately 6 hours. The dried ZIF-67 particles were added to ethanol to a concentration of 2% by mass and dispersed for 10 minutes using an ultrasonic homogenizer to prepare an ethanol dispersion of ZIF-67 particles.

[0033] This ethanol dispersion of ZIF-67 particles was sprayed onto the Au electrode surface of the QCM element 20 times using an electrostatic spray method at an applied voltage of 12 kV, and the resulting coating was then heated in a vacuum sample dryer at 120°C for approximately 1 hour and vacuum dried to produce a QCM element (gas sensor) with a dried coating on the Au electrode surface.

[0034] <Electron microscope observation> The cross sections of the dried coating films of the gas sensors fabricated in Examples 1 and 2 and Comparative Example 1 were observed using a scanning electron microscope ("SU3500" manufactured by Hitachi High-Technologies Corporation). Furthermore, for the gas sensor fabricated in Comparative Example 1, the surface of the dried coating film was also observed using a scanning electron microscope. Figures 1 and 2 show the cross sections of the dried coating films of the gas sensors fabricated in Examples 1 and 2. Furthermore, Figure 3 shows the surface (a) and cross section (b) of the dried coating film of the gas sensor fabricated in Comparative Example 1.

[0035] As shown in Figure 3, the dry thin film (Comparative Example 1) formed by applying a dispersion of ZIF-67 particles was confirmed to be formed by the deposition of ZIF-67 particles. On the other hand, as shown in Figures 1 and 2, the dry thin films (Examples 1 and 2) formed by applying a precursor solution of the ZIF-67 structure were found to be extremely dense layered films that were not recognizable as particles at the macro level. Furthermore, it was found that these dry thin films accurately followed the unevenness of the Au electrode and were well surface-bonded to the Au electrode. Furthermore, it was confirmed that the more concentrated (higher viscosity) the precursor solution, the thicker the dry thin film formed.

[0036] <Imidazole compound content> First, the mass of the dried coating film of the gas sensors prepared in Example 2 and Comparative Example 1 was measured. Specifically, the mass of the QCM element before film formation and the mass of the gas sensor after film formation were measured, and the difference was taken as the mass of the dried coating film. The results are shown in Table 1.

[0037] Next, the gas sensor was immersed in 500 μl of ethanol and subjected to ultrasonic treatment for 10 minutes to extract the imidazole-based compounds contained in the dried coating film. The obtained extract was analyzed using a gas chromatograph mass spectrometer (GC / MS). The results are shown in Figure 4. As shown in Figure 4, no peaks derived from imidazole-based compounds were observed in the dried coating film of the gas sensor prepared in Comparative Example 1. On the other hand, peaks derived from imidazole-based compounds were observed in the dried coating film of the gas sensor prepared in Example 2.

[0038] Based on the gas chromatogram shown in FIG. 4, the mass of the imidazole-based compound contained in the dried coating film was determined. Further, from the mass of the dried coating film and the mass of the imidazole-based compound, the content rate of the imidazole-based compound in the dried coating film was calculated. The results are shown in Table 1.

[0039]

Table 1

[0040] As shown in Table 1, the dried coating film of the gas sensor prepared in Example 2 contained 9.3% by mass of 2-methylimidazole, whereas the 2-methylimidazole contained in the dried coating film of the gas sensor prepared in Comparative Example 1 was only 0.3% by mass.

[0041] <X-ray Diffraction Measurement> The X-ray diffraction spectrum of the dried coating film of the gas sensor prepared in Example 2 was measured. The results are shown in FIG. 5. For reference, FIG. 5 shows the X-ray diffraction pattern of known ZIF-67 particles. As shown in FIG. 5, the X-ray diffraction pattern of the known ZIF-67 particles reflects only the clear peaks of the ZIF-67 crystals, whereas the dried coating film of the gas sensor prepared in Example 2 shows a halo pattern in the X-ray diffraction pattern together with the peaks of the ZIF-67 crystals, indicating that it contains an amorphous ZIF-67 structure together with the ZIF-67 crystal structure. Also, based on the X-ray diffraction pattern shown in FIG. 5, the following formula: Crystallinity (%) = Crystalline peak area / (Crystalline peak area + Amorphous peak area) × 100 was used to determine the crystallinity of the dried coating film, and it was found to be 29%. From this result, it was found that 71% of the ZIF-67 structures in the dried coating film of the gas sensor prepared in Example 2 are amorphous structures.

[0042] From the above results, it was found that the dry coating film of the gas sensor prepared in Example 2 was a composite film containing a dense ZIF-67 structure mainly composed of amorphous ZIF-67 and an imidazole-based compound. On the other hand, it was found that the dry coating film of the gas sensor prepared in Comparative Example 1 was essentially composed of ZIF-67 particles only.

[0043] <Gas sensing characteristics> The gas sensing characteristics of the gas sensors prepared in Example 2 and Comparative Example 1 were evaluated by the following method. That is, the gas sensor was fixed to a probe head for gas phase measurement and electrically connected to a QCM measurement device ("QCM922A" manufactured by Seiko EG&G Co., Ltd.). Nitrogen gas or acetone-containing nitrogen gas (acetone concentration: 1 ppm or 2 ppm) was supplied to the probe head for gas phase measurement at a flow rate of 500 cm3, with the gas sensor being switched at regular intervals. 3 The gas sensor was allowed to flow at a rate of 1 / min, and the changes in resonant frequency and resonant resistance were measured using a QCM measuring device. The results are shown in Figures 6 and 7.

[0044] As shown in Figure 6, when the gas sensor (Example 2) equipped with the dry thin film formed by applying the precursor solution of the ZIF-67 structure was used, it was confirmed that the resonant resistance did not fluctuate with the change in the acetone concentration in the flowing gas, and a stable signal was obtained. Furthermore, it was confirmed that the resonant frequency reversibly and accurately changed in response to the change in the acetone concentration in the flowing gas. Therefore, it was confirmed that the gas sensor of the present invention equipped with the dry thin film formed by applying the precursor solution of the ZIF-67 structure can stably detect gas components in the gas phase.

[0045] On the other hand, as shown in Figure 7, the gas sensor (Comparative Example 1) equipped with a dry thin film formed by applying a dispersion of ZIF-67 particles showed fluctuations in resonance resistance with changes in the acetone concentration in the flowing gas, indicating that the signal from the sensor was unstable. It was also found that the resonance frequency did not respond accurately to changes in the acetone concentration in the flowing gas. In other words, it was found that the gas sensor equipped with a dry thin film formed by applying a dispersion of ZIF-67 particles showed unstable detection of gas components in the gas phase.

[0046] In addition, the gas sensor (Example 2) equipped with a dry thin film formed by applying a precursor solution of the ZIF-67 structure can stably detect gas components in the gas phase, while the gas sensor (Comparative Example 1) equipped with a dry thin film formed by applying a dispersion of ZIF-67 particles is capable of stably detecting gas components in the gas phase. The reason for this is not entirely clear, but the inventors speculate as follows. That is, in the gas sensor prepared in Comparative Example 1, as shown in FIG. 3, a dry thin film made of ZIF-67 particles is formed on the electrode surface, and the contact between the electrode surface and the ZIF-67 particles and between the ZIF-67 particles are point contacts. Therefore, the contact area between the electrode surface and the ZIF-67 particles is insufficient, and the bonding strength between the electrode surface and the ZIF-67 particles and between the ZIF-67 particles is weak. As a result, the ZIF-67 particles vibrate randomly within the dry thin film in a resonant state, and this vibration becomes noise, making the signal from the sensor unstable, which is presumably causing unstable detection of gas components in the gas phase. In contrast, in the gas sensor prepared in Example 2, the imidazole-based compound present in the dry thin film increases the adhesion between the dry thin film and the electrode surface, resulting in a surface-to-surface contact between the electrode surface and the dry thin film, as shown in Figure 2. This stabilizes the signal from the sensor and is presumably what enables stable detection of gas components in the gas phase. [Industrial Applicability]

[0047] As described above, according to the present invention, the adhesion between the sensitive film and the surface of the transducer and the adhesion between the ZIF amorphous particles are excellent, so the signal from the sensor is stable, and it is possible to stably detect even trace amounts of gas components (for example, gas concentration: tens of ppm or less).

[0048] Therefore, the gas sensor of the present invention is particularly useful as a quartz crystal microbalance (QCM) gas sensor that detects a change in resonant frequency due to an increase in the mass of the sensitive film caused by gas adsorption to the ZIF in the sensitive film.

Claims

1. A gas sensor comprising a transducer and a sensitive film disposed on a surface of the transducer, A gas sensor characterized in that the sensitive film is formed of a zeolite-imidazole structure and further contains more than 0.3 mass % of an imidazole-based compound.

2. 2. The gas sensor according to claim 1, wherein at least a part of the zeolite-imidazole structure has an amorphous structure.

3. 2. The gas sensor according to claim 1, wherein the crystallinity of the zeolite-imidazole structure determined based on the XRD diffraction pattern of the sensitive film is 0 to 29%.

4. A step of mixing an imidazole compound and an acetic acid hydrate containing a metal ion in an organic solvent so that the molar ratio (imidazole compound / acetic acid hydrate containing a metal ion) is 1 / 1 or more to prepare a precursor solution of a zeolite-imidazole structure; a step of applying a precursor solution of the zeolite-imidazole structure to a surface of a transducer to form a coating film; a step of converting the precursor of the zeolite-imidazole structure into a zeolite-imidazole structure while removing the organic solvent in the coating film, thereby forming a sensitive film formed of the zeolite-imidazole structure; 2. A method for manufacturing a gas sensor, comprising:

5. 5. The method for manufacturing a gas sensor according to claim 4, wherein in the step of forming the zeolite-imidazole structure, the organic solvent in the coating film is removed by heating at a temperature equal to or higher than the boiling point of the organic solvent and lower than the boiling point of the imidazole-based compound.

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